This invention relates generally to braking systems, and more particularly concerns closed-center braking systems for agricultural tractors and like devices.
The use of hydraulic systems on farm tractors and like implements is important to tractor manufacturers and associated organizations. Experience with these systems has shown the desirability of having a brake system and a brake actuator valve having good pressure modulation capability, reliability, and easy serviceability. Generally accepted practice has dictated, until now, that these brake actuator valves be of a spool-type design. However, under at least some circumstances, these systems and valves have provided less than perfect pressure modulation as system hydraulic fluid flow rates vary.
Further, even when valve parts are carefully polished and fitted, internal leakage within the brake valve can, under some circumstances, become a matter of concern. Moreover, the practice of so carefully fitting valve parts has occasionally led to later difficulties in field servicing.
For some time, braking and associated hydraulic systems used in some tractors have utilized a "closed-center" concept. Briefly, these systems utilize a pump which draws fluid from a reservoir and provides, at the pump head, a constant fluid pressure; the rate of fluid flow through the system can vary from nil to whatever amount may be necessary at the time to operate down stream system mechanisims.
It is the general object of the present invention to provide a valve for use with such a closed-center brake system which will improve the performance and operating characteristics of the entire system. A related object is to provide such a valve system which eliminates the expense and reliability problems of spool-type internal valve mechanisms.
Another object is to provide a valve and related braking system in which internal leakage problems are decreased, if not entirely eliminated. An ancillary object is to provide such a valve which is subjected to part wear action to only a limited degree, and which will, in consequence, provide a long valve service life.
Yet another object is to provide a valve and system of the type described which can be offered at low initial cost, and which can be maintained throughout its useful life at a relatively modest service cost.
Another object is to provide a valve for use in a closed-center brake system of the type described which can provide for manual operation should the pump or other system components lose the ability to generate or sustain hydraulic pressure for some reason.
Still another object is to provide a brake valve and a related closed-center braking system which offers independent bracking action to the left and right wheel brakes.
A further object is to provide a valve and associated closed-center brake system which will automatically permit hydraulic operation of an auxiliary system element, such as a tractor differential, upon operation of even one of two brake actuators.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. Throughout the drawings, like reference numerals refer to like parts.